1. Field of the Invention
The present invention relates to color processing of generating a color separation table.
2. Description of the Related Art
A printer forms an image by converting RGB signal values or CMYK signal values into the color material signal values, each of which corresponds to each color material held by the printer, and applying, to a printing medium, each color material, an amount of which corresponds to the color material signal value. The conversion into color material signal values is called color separation. A lookup table (LUT) and an interpolation technique such as tetrahedron interpolation or cubic interpolation are generally used for the color separation.
A lookup table (to be referred to as a “color separation table” hereinafter) used for color separation stores color material signal values corresponding to input signal values as an LUT for an N-grid. Note that if an input signal value includes eight bits for each color, N is an integer which falls within the range from 2 to 255. Color reproduction by the printer is based on the color separation. As a method of generating a color separation table, various methods have been proposed.
For example, Japanese Patent No. 2554366 (literature 1) discloses a method of forming patch images for which the color material values of C, M, Y, and K have been quantized, and predicting reproduction colors for the respective color material values using the colorimetric values of the patch images. Furthermore, Japanese Patent Laid-Open No. 6-242523 (literature 2) discloses a method of performing color separation for a target color to obtain C, M, Y, and K based on the predicted reproduction colors for the respective color material values, which are obtained by the method of literature 1.
Those techniques in literatures 1 and 2 can improve the tonal continuity by combining a color represented by the input signal value and the colors reproduced based on the color material signal values but cannot control the graininess of a print image.
On the other hand, Japanese Patent No. 4623029 (literature 3) discloses an operation of controlling the graininess using representative tones in an RGB color solid. According to literature 3, the representative tones include a tone from a white point Pw to a black point Pk, and tones from the point of red (R), green (G), blue (B), cyan (C), magenta (M), or yellow (Y) to the black point Pk. For each tone, the ratio of under color removal (UCR ratio) of the tone values of achromatic colors each represented by a combination of chromatic color materials, which are replaced by a black color material, is set, and the UCR ratio of a point within a color gamut is interpolated by a method shown in FIG. 20 of literature 3, thereby setting the UCR ratio according to the hue and chroma.
In recent years, there are many printers having color materials with the same hue and different densities to improve the graininess. Representative examples are light color materials such as light cyan (LC) and light magenta (LM) each having a low density.
Japanese Patent No. 4035278 (literature 4) relates to generation of a color separation table using light cyan (LC) and light magenta (LM), and discloses an operation of controlling the graininess using representative tones in an RGB color solid. According to literature 4, the representative tones include a tone from a white point Pw to a black point Pk, and tones from the white point Pw to the black point Pk through the point of red (R), green (G), blue (B), cyan (C), magenta (M), or yellow (Y). For each tone, color material switching occurs. For example, in transition from Pw to C, switching from light cyan (LC) to cyan (C) occurs. A point (to be referred to as an “application start point” hereinafter) of starting to apply cyan (C) in the color material switching is set, and the application start point of cyan (C) within the color solid is calculated by a color material value interpolation technique based on color material values.
In the graininess control operation disclosed in literature 3 or 4, even if the graininess of the representative tones can be controlled with high accuracy, the color material value of a point within the color solid is determined by interpolation based on color material values, and is not always optimal. The graininess indicates a fluctuation in density on an image surface. In the light of color separation for reproducing color by area coverage modulation, if the local density of a single dot of a color material is relatively high with respect to the global density for an arbitrary area, granularity is observed. That is, it is not ensured that an interpolated color material value always has the same or higher density (or lower lightness).
As a result, a black color material which is controlled according to the hue/chroma by the technique of literature 3 may be used in a region where the density is lower (lightness is higher) than that used in the representative tones. Furthermore, in the technique of literature 4, a color material with a relatively high density may be used in a region where the lightness is higher than that used in the representative tones.